This invention relates to a resource-saving, pollution-free method for the production of refined aluminum or aluminum alloys. (Hereinafter, aluminum and aluminum alloys shall, as a rule, be collectively referred to as "aluminum".) More particularly, this invention relates to a method for refining magnesium-containing aluminum by removing magnesium therefrom and to a method for producing aluminum of high purity by refining impure aluminum.
These methods are required to avoid creating environmental pollution particularly in the removal of magnesium and to permit economization of resources particularly in the production of refined aluminum. These requirements are fulfilled by the methods of the present invention.
First, the removal of magnesium will be described.
The treatment of aluminum for the removal of magnesium therefrom is a refinement process most necessary for the recycling of aluminum.
Magnesium, if contained in wrought products of aluminum alloy (containing pure aluminum), improves the mechanical properties of the material without impairing the corrosion-resistance thereof. In this respect, it is a useful alloying element. Aluminum alloys prepared so as to contain up to about 5% of magnesium find widespread acceptance as corrosion-resistant aluminum alloys.
In the case of aluminum alloys for use in die castings, however, magnesium is an unwanted element. Inclusion of magnesium in these aluminum alloys, therefore, is rigidly controlled. For use in castings, aluminum alloys are prepared so as to contain a fairly large proportion of silicon with a view to improving their casting property. Magnesium, if present therein, reacts with silicon to produce an intermetallic compound Mg.sub.2 Si which embrittles the alloys. This explains why the presence of magnesium in such aluminum alloys is undesirable.
According to JIS (Japanese Industrial Standard) H5302 titled "Die Casting Aluminum Alloys," the allowable magnesium content in ADC10 and ADC12, which together account for more than 95 percent of the die casting alloys actually used in Japan, is specified as not to exceed 0.3 percent. Most standards adopted by ordinary consumers and those adopted by producers dictate still lower magnesium contents in aluminum alloys. In the case of 380-series alloys which are used in the largest quantities in the United States, the highest allowable magnesium content is 0.10 percent.
About 90 percent of all the die casting alloys are produced by using aluminum scraps as the raw material. In wrought products of aluminum alloys produced today, the average magnesium content is 0.96 percent. It follows as a consequence that the content of magnesium in aluminum scraps generally exceeds the allowable upper limit of magnesium content in die casting alloys. For use in the preparation of die casting alloys, therefore, such aluminum scraps must be given a treatment for removal of magnesium. The processes heretofore employed for the removal of magnesium are such that they have inevitably caused air pollution.
The removal of magnesium from alluminum alloys has heretofore been accomplished exclusively by either of two methods: (1) one method using chlorine gas and (2) another using a fluoride. The former method comprises blowing chlorine gas into a given molten aluminum alloy. Magnesium exhibits a stronger chemical affinity for chlorine than aluminum. By making use of this difference of affinity, magnesium is removed in the form of MgCl.sub.2 from the aluminum alloy. The waste gas emanating from this treatment, consequently, contains unaltered free chlorine and aluminum chloride in large proportions and causes air pollution. When chlorine gas is blown into the molten aluminum alloy through a graphite pipe as is usually practiced, the chlorine concentration in the waste gas reaches the level of about 3000 ppm. To lower the chlorine concentration in the waste gas, there have been studied various measures, including those of Bell System Process, Alcoa Process, Derham Process, etc.
With the demagging method which makes use of chlorine gas, however, the waste gas is certained to contain harmful free chlorine and aluminum chlorine.
In the method which resorts to use of a fluoride, aluminum fluoride is mainly used as the fluoride and the reaction shown below is utilized for the removal of magnesium. EQU 2AlF.sub.3 +3Mg=3MgF.sub.2 +2Al
This compound, aluminum fluoride, is so expensive as to render this method uneconomical. Moreover, aluminum fluoride undergoes decomposition to give rise to a fluorine compound which causes air pollution. For this reason, this method is not used so widely as the aforementioned method which effects the treatment by use of chlorine.
The inventor of this invention formerly invented a method for effecting removal of magnesium from aluminum without use of any chlorine gas or aluminum fluoride. To this invention have issued Japanese Pat. No. 1,040,009 and U.S. Pat. No. 4,183,745. This method obviates the necessity for using chlorine gas or aluminum fluoride and enables magnesium contained in aluminum, a valuable substance which has heretofore been discarded in the form of magnesium chloride or magnesium fluoride, to be recovered in the form of metallic magnesium. It is, therefore, a resources-saving, pollution-free method.
FIG. 1 represents a typical apparatus used for working the invention of U.S. Pat. No. 4,183,745. In a refractory container 1, molten aluminum bath 2 containing magnesium is contained and an electrolytic bath 3 which is a fused flux layer containing chlorides and/or a fluoride is superposed on the molten aluminum bath 2. The molten aluminum bath 2 is provided with an anode and the electrolytic bath 3 with a cathode respectively. The first problem encountered by this method resides in the fact that "threshold current density" exists on the surface of the molten aluminum bath while the apparatus is in an energized state. When the current density exceeds this threshold, not only magnesium but also aluminum reacts with chlorine and forms aluminum chloride on the surface of the molten aluminum bath. The aluminum chloride thus formed passes into the electrolytic bath 3. On reaching the interior of the electrolytic bath 3, the aluminum chloride which has a lower decomposition voltage than magnesium chloride immediately separates itself and adheres as a deposit 4 to the cathode, giving birth to an alloy of magnesium and aluminum on the cathode. As the aluminum content increases in this alloy of magnesium and aluminum, the alloy gradually gains in specific gravity. When the increased specific gravity of the alloy surpasses the specific gravity of the electrolytic bath 3, the alloy sediments to the lower portion of the electrolytic bath. Consequently, the alloy passes into the molten aluminum bath 2 which is undergoing the treatment for the removal of magnesium and completely spoils the treatment for the magnesium removal. The second problem of this method resides in the fact that an attempt to recover magnesium in the form of pure metal magnesium only results in heavy oxidative consumption of molten magnesium and consequent notable decline of the ratio of recovery.
Now, the method for producing aluminum refined to high purity will be described below. The term "high-purity aluminum" generally refers to aluminum which has purity of at least 99.95% and is suitable for use such as in electrolytic condensers. Heretofore, the high-purity aluminum has been produced by the method of three-phase electrolysis using, as an anode, a molten aluminum alloy which possesses a high specific gravity because of incorporation of about 33% of copper therein. This three-phase electrolysis method has disadvantages such as entailing a high unit power consumption on the order of 18000 to 20000 KWH, involving a high unit consumption of aluminum metal on the order of 1030 to 1050 kg, and inevitably entailing heavy consumption of copper on the order of 5 to 10 kg per ton because the electrolysis requires use of an aluminum alloy with high specific gravity due to incorporation of 30 to 35% of copper as an anode.
An object of this invention is to provide a method for the removal of magnesium from magnesium-containing aluminum, which method avoids creating the cause for environmental pollution and permits magnesium to be recovered in high efficiency.
Another object of this invention is to provide a method for the production of refined aluminum with a low unit power consumption and a low unit aluminum consumption without requiring use of copper.